This invention relates to a linkage adjuster for a drum brake for adjusting the braking-induced wear at friction linings having an overload clutch and a one-way rotational clutch or directional clutch.
In heavy utility vehicles, pneumatically actuated drum brakes are often used as friction brakes. A preferred type of construction is the so-called S-cam drum brake.
In order to generate a braking force, the brake pads of the drum brake, which form or support the brake linings, must be pressed in the radial direction from the inside against the drum of the drum brake. In the S-cam drum brake, this takes place by rotating a brake shaft, which has, at one end, a double involute in an S-shape. The two brake pads are supported with their ends on the cam, with the ends that are situated opposite these ends being fastened, for example, to a brake carrier so as to be pivotable about a bolt. For braking, the brake shaft with the S-cam must be rotated until the brake pads come into contact against the drum with the desired force. This rotation of the brake shaft takes place by way of a compressed-air-actuated brake cylinder, which, with its piston, generates a torque by way of a lever and transmits that torque to the brake shaft.
Since the brake pads or the brake linings become worn during braking, it is necessary to compensate for the brake lining wear with an alignment device. For this purpose, the brake shaft with the S-cam must be rotated by a corresponding magnitude in the direction in which the brake linings are adjusted towards the drum during the braking process, in order to move the brake pads closer to the inside of the drum. This direction is referred to as the application direction. However, in the rest position, a small gap must remain in order to ensure an efficient release behavior. Re-alignment can be carried out at regular intervals, and also manually. For this purpose, re-alignment must be carried out at the brake lever of the brake shaft on which the brake cylinder engages with its piston rod. Since manual adjustment is carried out neither according to requirements nor continuously, so-called automatic linkage adjusters are standard equipment on vehicles with S-cam drum brakes.
There are various linkage adjuster systems. A first classification can be made according to the adjusting principle. According to one adjusting principle, adjustment takes place at the start of the braking process, while, according to the other adjusting principle, adjustment is carried out at the end of the braking process, or during the release process.
Further differences relate to the type of alignment of the control disk. In order to permit the adjustment function, the control disk must be so rotationally fixedly connected to an axle part that a relative rotational or pivoting movement can take place between the control disk and the linkage adjuster. Here, it is to be ensured that, during installation of the linkage adjuster, the control disk is aligned within a previously defined working range.
In order to avoid faults, linkage adjusters have therefore also been developed with so-called self-alignment. Here, the control disk must be connected only to the axle, and alignment is no longer necessary.
A linkage adjuster with a basic design is composed substantially of a worm gear, a worm, an overload or load shift clutch, a preload spring, a one-way clutch (usually operating on the wrap spring principle), and, in the case of manual alignment, a gearwheel, a toothed rack, and a so-called control disk with a notch, or in the case of automatic adjustment, a further worm gear, a drive input worm with a gearwheel, and a control disk with an outer toothing.
The mechanism is situated in a housing having a lever to which the brake cylinder is articulatedly connected. The brake shaft is coupled by way of a spline toothing to the worm gear. The control disk, which is rotatable relative to the linkage adjuster, is connected, as mentioned above, to the axle.
Reference is made as an illustrative example to European document EP 0 614 025 B1. This document describes an adjusting device for a disk brake, with both of the above-stated functional principles being used together by way of adjustment carried out at the start of the braking process and tensioning an elastic element. The elastic element is relaxed during the release process and brings about a further adjustment.
Linkage adjusters that are presently on the market do not always meet the expectations or demands with regard to durability and permanent functionality. A one-way clutch in the form of a wrap spring has been proven to be a component which is susceptible to wear. The function of this component is highly dependent on the friction conditions, on the shaping, and on the dimensional stability of the mating faces. If wear and/or changes to the friction conditions occur over the course of time, failure of the component, and therefore of the entire linkage adjuster, can occur.
In addition, the mechanical expenditure for linkage adjusters (in particular with self-alignment) is relatively high, since a plurality of individual parts of precision design are required for construction of those adjusters.
An object of the present invention is to create a linkage adjuster of the above-specified type with a mechanism that has a high level of robustness and a simple construction, with the linkage adjuster additionally being provided with self-alignment. A further object is to provide the most cost-effective production possible.
According to the invention, therefore, a linkage adjuster for a drum brake for adjusting braking-induced wear at friction linings includes an overload clutch and a one-way rotational clutch or directional clutch. At least one of the overload and one-way rotational or directional clutches includes at least one of a ball ramp clutch, which is preloaded by a spring, and a cone clutch.
With a linkage adjuster according to the present invention, it is possible for the above-described difficulties or disadvantages to be avoided, since the linkage adjuster has a low number of individual parts, construction takes place with simple-to-produce individual parts, and the individual parts have a high level of wear resistance.
A linkage adjuster according to the invention, for a drum brake adjusting for braking-induced wear at friction linings, has an overload clutch and a one-way rotational clutch or directional clutch, and includes a ball ramp clutch, which is preloaded by a pressure spring, and a cone clutch.
In another embodiment, the linkage adjuster has a construction with two modules. One module has the ball ramp clutch and the cone clutch, and the other module preferably performs the directional clutch function.
In a further preferred embodiment, the linkage adjuster has two modules, with one module having the overload clutch and the other module having the ball ramp clutch and the cone clutch with directional clutch, manual restoring, and force transmission functions. An advantageous functional distribution is therefore possible, with the two modules arranged in a simple construction with a low susceptibility to faults.
Preferably, the ball ramp clutch is composed of oppositely arranged end faces, each of which is formed with a ball ramp contour for clamping balls arranged in between. Here, it is particularly advantageous for the components to be mounted by ball bearings, since a high level of efficiency is permitted in this way.
By way of simple production, the ball ramp contours of the end faces of the ball ramp clutch have different gradient angles. This is particularly advantageous if the first gradient angle of the ball ramp contour, which forms the drive input side during adjustment, is smaller than the second gradient angle of the ball ramp contour, which forms the drive output side during adjustment, since a directional clutch is permitted in this way.
In an alternative embodiment, the ramp contours have curved formations with different radii of curvature.
The balls provided between the ramp contours are arranged in a ball cage that is preloaded by the pressure spring in such a way that the balls, during adjustment, bear against the ball ramps for force transmission. It is preferable for the spring or pressure spring to be embodied as a friction spring. This results in a precise functional behavior on account of the use of precise and friction-independent components.
In one preferred embodiment, the overload clutch of one module is formed by a control disk, which is connected in a frictionally engaging manner to a stationary locking ring. Here, it is advantageous for the control disk to be arranged between two friction disks, which are preloaded by a plate spring. The load of the plate spring can be easily defined in advance by its construction. In addition, the required parts are few in number, simple to produce, and easy to assemble.
A drum brake according to the invention has a linkage adjuster according to the invention.
Further advantages, details and features of the invention can be gathered from the following detailed description in connection with the appended drawings.
The invention is explained in more detail below on the basis of the exemplary embodiment illustrated in the drawing figures.